The progressive 2e−/2H+ reduction of the porphyrinic macrocycle along the series porphyrin, chlorin (a dihydroporphyrin) and bacteriochlorin (a tetrahydroporphyrin) causes profound changes in chemical and physical properties (Scheme 1). The reduction alters the symmetry yet each macrocycle maintains an 18 π-electron conjugated system as required for aromaticity. One striking change upon reduction is the large increase in absorption in the red or near-IR region of the spectrum.

The changes in physical properties have been famously exploited by biological systems; the chlorin macrocycle provides the basis for chlorophyll a and b in plant photosynthesis while the bacteriochlorin macrocycle provides the basis for bacteriochlorophyll a in bacterial photosynthesis. The striking change in absorption is illustrated for a representative porphyrin, chlorin, and bacteriochlorin in FIG. 1. (Sternberg, E. D.; Dolphin, D. Tetrahedron 1998, 54, 4151-4202)
Two distinct types of bacteriochlorins occur in Nature, bacteriochlorophylls (type a, b, or g) and tolyporphins (A-J). The bacteriochlorophylls serve as the principal light-absorbing pigments and energy/electron-transfer components in bacterial photosynthetic systems. Bacteriochlorophyll a is the most widely distributed bacteriochlorin pigment and was the first bacteriochlorophyll isolated as a pure compound (Scheer, H. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. I, p 31; Scheer, H.; Inhoffen, H. H. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. I, p 45). Tolyporphin A, a non-photosynthetic bacteriochlorin pigment, was isolated from the cyanophyte microalga Tolypothrix nodosa in 1992, and a number of additional tolyporphins that contain the bacteriochlorin system have since been isolated (Prinsep, M. R. et al., J. Am. Chem. Soc. 1992, 114, 385-387; Prinsep, M. R. et al., Tetrahedron 1995, 51, 10523-10530; Prinsep, M. R. et al., J. Nat. Prod. 1998, 61, 1133-1136). The structures of bacteriochlorophyll a and tolyporphin A are shown in Scheme II.

Surprisingly few methods exist for the preparation of bacteriochlorins despite the importance of this class of compounds (Johnson, A. W.; Oldfield, D. J. Chem. Soc. 1965, 4303-4312; Dinello, R. K.; Dolphin, D. J. Org. Chem. 198.0, 45, 5196-5204; Chang, C. K.; Sotriou, C. J. Org. Chem. 1987, 52, 926-929; Kozyrev, A. N. et al., Tetrahedron Lett. 1996, 37, 3781-3784; Shea, K. M. et al., Tetrahedron 2000, 56, 3139-3144). With regard to the naturally occurring bacteriochlorins, the total synthesis of the O,O-diacetate of tolyporphin A was reported several years ago by Kishi, entailing >20 steps and affording <5 mg of product (Wang, W.; Kishi, Y. Org. Lett. 1999, 1, 1129-1132). To our knowledge, no total syntheses of bacteriochlorophyll a have been reported. A chief obstacle to handling bacteriochlorophyll a is its pronounced tendency to undergo dehydrogenation to give the corresponding chlorin. The same tendency for oxidative reversion to the chlorin or porphyrin occurs with bacteriochlorins that have been prepared by hydrogenation of the porphyrin or chlorin (Dorough, G. D.; Miller, J. R. J. Am, Chem. Soc. 1952, 74, 6106-6108; Whitlock, H. W. et al., J. Am. Chem. Soc. 1969, 91, 7485-7489; Fajer, J. et al., Proc. Nat. Acad. Sci. USA. 1974, 71, 994-998; Bonnett, R. et al., Biochem. J. 1989, 261, 277-280; Grahn, M. F. et al., J. Photochem. Photobiol. B: Biol. 1997, 37, 261-266; Senge, M. O. et al., S. Tetrahedron 1998, 54, 3781-3798). More resilient bacteriochlorins have been prepared by derivatization of porphyrins or chlorins via vicinal dihydroxylation (typically followed by pinacol rearrangement for porphyrins that bear β-substituents) (Chang, C. K. et al., J. Chem. Soc. Chem. Commun. 1986, 1213-1215; Adams, K. R. et al., J. Chem. Soc. Perkin Trans. 11992, 1465-1470; Pandey, R. K. et al., Tetrahedron 1992, 51, 7815-7818; Kozyrev, A. N. et al., Tetrahedron Lett. 1996, 37, 3781-3784; Pandey, R. K. et al., J. Med. Chem. 1997, 40, 2770-2779; Pandey, R. K. et al., J. Org. Chem., 1997, 62, 1463-1472; Zheng, G. et al., J. Org. Chem. 1999, 64, 3751-3754; Chen, Y. et al., J. Org. Chem. 2001, 66, 3930-3939; Li, G. et al., J. Org. Chem. 2004, 68, 3762-3772), Diels-Alder reaction (Tomé, A. C. et al., Chem. Commun. 1997, 1199-1200; Vincente, M. G. H. et al., Chem. Commun. 1998, 2355-2356; Cavaleiro, J. A. S. et al., J. Heterocyclic Chem. 2000, 37, 527-534), or 1,3-dipolar cycloaddition (Silva, A. M. G. et al., Tetrahedron Lett. 2002, 43, 603-605). While each of the derivatization methods has merit, a key limitation lies in the formation of regioisomers upon use of porphyrinic substrates bearing a distinct pattern of substituents. On the other hand, modification of naturally occurring bacteriochlorophylls can yield elaborate bacteriochlorin derivatives, but the presence of a nearly full complement of peripheral substituents in the naturally available starting materials restricts synthetic flexibility (Mironov, A. F et al., J. Porphyrins Phthalocyanines 2003, 7, 725-730; Mironov, A. F. et al., Russ. J. Bioorg. Chem. 2003, 29, 190-197; Hartwich, G. et al., J. Am. Chem. Soc. 1998, 120, 3675-3683; Tamiaki, H. et al., Tetrahedron: Asymmetry 1998, 9, 2101-2111; Wasielewski, M. R. et al., J. Org. Chem. 1980, 45, 1969-1974).